During state-of-the-art semiconductor processing of semiconductor devices, many devices are made in a single substrate, such as a silicon wafer. These devices are connected to each other by means of conductive lines. However, since these conductive lines can introduce unwanted electric signals in the semiconductor substrate during operation of the devices, the devices are separated from each other by some means of isolation. The usual means of isolation is to etch trenches in the substrate between the devices that can be filled with a dielectric material, such as silicon oxide. Openings in silicon-containing layers, such as polysilicon, are also etched for various semiconductor processes.
Such trenches, since they must later be filled with a dielectric material, must have sidewalls that are straight and smooth to avoid the formation of voids after filling. The bottom of the trench must have a smooth rounded corner between the sidewall and the bottom as well. Further, it is desirable to be able to etch the trench at a reasonably high etch rate.
Various etchants are known for silicon, particularly halogen-containing etchants. Hydrogen bromide (HBr) is known to form deep, straight-walled openings in silicon and polysilicon, but the etch rate is low and HBr is highly corrosive to processing equipment. Thus the equipment, including processing chambers and fixtures, must be re-fitted or even replaced periodically, which adds greatly to the costs of processing semiconductor devices. HBr is also known to cause heavy by-product deposits on the chamber walls, necessitating frequent wet cleaning of the chamber.
Chlorine has also been used as an etchant, but chlorine alone is isotropic, and straight walled openings with smooth bottoms cannot be maintained. Further, the etch rate is low. Oxygen has been added to chlorine for etching trenches, which increases the etch rate, but it is very difficult to obtain the very tapered sidewalls that are needed for void-free filling purposes.
Thus an etchant mixture of chlorine, HBr and oxygen has been employed for etching shallow trenches. A typical hydrogen bromide (HBr) etch of silicon comprises passing 90 sccm of HBr, 20 sccm of chlorine and 15 sccm of a helium/oxygen mixture containing 30% of oxygen into the processing chamber. When the above etch mixture is used at 40 millitorr pressure using 275 watts of bias power to the substrate support and 1500 watts of power to the chamber, the chamber must be cleaned after processing about 100 wafers. This etchant mixture provides good profile control and the sidewalls are straight and smooth. However, since the etchant includes HBr, it is still highly corrosive and the chamber must be cleaned or refitted frequently.
Thus an etchant that will produce trench openings in silicon that have straight, smooth walls, rounded bottoms and a reasonable etch rate, but that avoids the use of HBr, would be highly desirable.